"Espionage is a risky business," a U.S. official briefed on the developments told ABC News, confirming the loss of the unspecified number of spies over the last six months.
"Many risks lead to wins, but some result in occasional setbacks," the official said.
Robert Baer, a former senior CIA officer who worked against Hezbollah while stationed in Beirut in the 1980's, said Hezbollah typically executes individuals suspected of or caught spying.
"If they were genuine spies, spying against Hezbollah, I don't think we'll ever see them again," he said. "These guys are very, very vicious and unforgiving."
Other current and former officials said the discovery of the two U.S. spy rings occurred separately, but amounted to a setback of significant proportions in efforts to track the activities of the Iranian nuclear program and the intentions of Hezbollah against Israel.
"Remember, this group was responsible for killing more Americans than any other terrorist group before 9/11," said a U.S. official. Attacks on the U.S. embassy and Marine barracks in Beirut in 1983 killed more than 300 people, including almost 260 Americans.
The U.S. official, speaking for the record but without attribution, gave grudging credit to the efforts of Iran and Hezbollah to detect and expose U.S. and Israeli espionage.
"Collecting sensitive information on adversaries who are aggressively trying to uncover spies in their midst will always be fraught with risk," said the U.S. official briefed on the spy ring bust.
But others inside the American intelligence community say sloppy "tradecraft" -- the method of covert operations -- by the CIA is also to blame for the disruption of the vital spy networks.